Re: [PATCH 2/2] mm: prevent gup_fast from racing with COW during fork

[Jason Gunthorpe <jgg@nvidia.com>]

On Mon, Oct 26, 2020 at 05:14:35PM -0700, Linus Torvalds wrote:
> On Mon, Oct 26, 2020 at 4:50 PM Jason Gunthorpe <jgg@nvidia.com> wrote:
> >
> > Right, it isn't a "seqcount" because the read side doesn't spin.
> 
> Not all readers spin for seqcount either.
> 
> Sure, it's the *common* case, but look at name resolution - it will
> basically fail on a seqcount failure in RCU mode, and go to the "slow
> and safe" version:
> 
>                 if (unlikely(read_seqcount_retry(&d->d_seq, nd->seq)))
>                         return -ECHILD;
> 
> Exactly like gup_fast vs fork, in fact.
> 
> So no, you don't need to retry for seqcount failures, doing
> 
>      seq = seqcount_begin(..);
>      ... optimistic fast case ...
>      if (!read_seqcount_retry(.., seq))
>           return 0; // success
>        .. do slow case ...
> 
> is valid and happens.

Okay, I see d_seq using one of the raw_write versions, so I'll make
this look like that

Thanks,
Jason